The quality of an image (to be referred to as an image quality hereinafter) by a recently popular image forming apparatus (to be referred to as a printer hereinafter) is determined by various factors such as graininess, in-plane uniformity, character quality, and color reproducibility (including color stability). A most important factor of them is color reproducibility.
As for color reproducibility, the color difference matters not only between devices of the same model, but also between devices of different models, between image forming apparatuses of different methods, and between an image forming apparatus and an image display apparatus. To perform color matching between such devices, software and a measurement device for generating a multidimensional LUT (Look Up Table) called an ICC (International Color Consortium) profile are commercially available.
As shown in FIG. 5, the contents of each ICC profile are calibrated in association with a device-independent color space based on color measurement of a measurement image (patch) using the measurement device. An example of the color space is a CIE L*a*b* color space (CIE stands for Commission International de l'Eclairage). By this processing, colors to be printed can match each other even between different devices. A CMM (Color Management Module) installed in an image forming apparatus or the like can generate print data by performing color conversion using these profiles.
Japanese Patent Laid-Open No. 2004-86013 proposes an inline measurement device arrangement in which a patch image formed on a sheet is detected by a color sensor constructed by a light source, diffraction grating, and position detection sensor, thereby increasing the detection accuracy. A detection value from the color sensor is converted into a spectral reflectance, and the spectral reflectance can be converted into a CIE Lab value in consideration of the tristimulus values and the like. The color detection accuracy of the color sensor in Japanese Patent Laid-Open No. 2004-86013 degrades owing to fluctuation factors such as output fluctuations of the light source upon a change of the environmental temperature. To solve this, there is a method of performing calibration using a white reference plate arranged at a position facing the color sensor, and correcting the detection value of the color sensor. As the calibration method, light reflected by the white reference plate is measured before or after executing color measurement of a patch image, and then calculation is performed based on the measurement value. Letting W(λ) be the quantity of light reflected by the white reference plate and P(λ) be the quantity of light reflected by the patch, a spectral reflectance calculation method in calculation using the white reference plate is given by:
                              spectral          ⁢                                          ⁢          reflectance          ⁢                                          ⁢                      R            ⁡                          (              λ              )                                ⁢                                          ⁢          of          ⁢                                          ⁢          patch                =                              P            ⁡                          (              λ              )                                            W            ⁡                          (              λ              )                                                          (        1        )            
The white reference plate reflects light of a measurement wavelength region at almost the same reflectance regardless of the wavelength. Thus, the quantity W(λ) of light reflected by the white reference plate can be considered to be equal to the quantity of light incident on the patch image. By measuring both the quantity (that is, W(λ)) of light incident on the patch image and the reflected light quantity P(λ), the spectral reflectance R(λ) of the patch can be calculated without the influence of output fluctuations of the light source.
The color sensor includes a line sensor in which a plurality of light-receiving elements are arrayed. The positional relationship between each element of the line sensor and each light wavelength may deviate owing to variations in part assembly or the like, and a color wavelength component may be erroneously detected in measurement. This phenomenon will be called a wavelength detection error. To prevent the wavelength detection error, for example, the relationship between each element of the line sensor and the wavelength of incident light may be adjusted by adjustment processing in the factory.
However, in the color sensor, a holding member which holds each part thermally expands upon a temperature change. Then, the positional relationship between parts deviates and the tint (L*a*b* value) to be detected changes, resulting in a measurement error. Letting Δλ be the wavelength detection error amount, the quantity W(λ) of light reflected by the white reference plate, the quantity P(λ) of light reflected by the patch, and the spectral reflectance R(λ) have a relationship given by:
                              spectral          ⁢                                          ⁢          reflectance          ⁢                                          ⁢                      R            ⁡                          (              λ              )                                      =                              P            ⁡                          (                              λ                -                Δλ                            )                                            W            ⁡                          (                              λ                -                Δλ                            )                                                          (        2        )            
As a method of obtaining Δλ, a method of obtaining the peak pixel moving amount detected by the line sensor is conceivable. In initial adjustment, a light-receiving pixel of the line sensor from which a highest peak output was obtained is stored based on the detection value of the line sensor. By detecting the moving amount of the peak output upon a temperature change, the wavelength shift amount can be detected. In this manner, a wavelength shift arising from the sensor can be corrected by detecting the wavelength detection error amount Δλ and solving equation (2).
However, the method of calculating the wavelength detection error amount Δλ by peak detection has the following problem. If a wavelength fluctuation of the light source and a wavelength detection error on the sensor side co-occur upon a temperature change, the wavelength shift amount of the light source varied with temperature of the light source and the wavelength detection error amount Δλ on the line sensor side coexist in the peak pixel moving amount detected by the line sensor. For this reason, only the wavelength detection error amount Δλ cannot be obtained. Since the wavelength detection error amount Δλ cannot be detected accurately, it cannot be corrected accurately, generating a measurement error.